[docs] CToSwift: Tweak wording around "overrides"
The previous wording referred to a method or property satisfying a
protocol requirement as an "override", which both Clang and the Swift
Clang Importer use internally sometimes. But most people who program
Objective-C (or Swift) don't think of this as an override, so this
document shouldn't use that language. (Thanks, Varun!)
